Well, I had something wrong overnight. Modem lights looked fine but the HH3 wouldn't connect to the 'net (this was ~2am).
In the end rebooted the HH3 with the power switch (as restarting via the web gui did nothing) and eventually it came back up and connected.
A speedtest this morning showed that the restart (or whatever) appears to have addressed my upload speed issue with it going up from 9Mbps to just under 18Mbps. Downstream seems to be working well at ~68Mbps.
Rebooting the HH3 won't have changed anything at all with this, as the sync is done via the VDSL router rather than the HH? Your initial problems might have been a result of the VDSL router re-sync'ing though.